The Suomi KP/-31 (Suomi-konepistooli or "Submachine-gun Finland", literally: "Machine-pistol Finland") was a submachine gun (SMG) of Finnish design used during World War II. It was a descendant of the M-22 prototype and the KP/-26 production model, which was revealed to the public in 1925. The Suomi-konepistooli KP/-31 is often abbreviated to Suomi KP.

The Suomi KP/-31 is regarded by many[3] as one of the most successful submachine guns of World War II, also the soon developed 71-round drum magazine was later copied and adopted by the Soviets for their PPD-40 and PPSh-41 submachine guns.[4] The accuracy of the Suomi was superior to that of the mass-produced PPSh-41,[4] thanks in part to a noticeably longer barrel, with the same rate of fire and the equally large magazine capacity.[4] The major disadvantage of the Suomi KP/-31 was its high production cost.

The M-22 and KP/-26 were made by Konepistooli Oy, founded by Master Armorer Aimo Lahti, Captain V. Korpela, Lieutenant Y. Koskinen and Lieutenant L. Boyer-Spoof. The Suomi KP/-31 was designed by Koskinen and Lahti.

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The Suomi KP/-31 went into serial production in 1931 by Tikkakoski Oy and most of these weapons were bought by the Finnish Defence Forces. The Finnish Defence Forces were equipped with about 4000 Suomi KP/-31 submachine guns when the Winter War started. During the course of the war, the design was altered with the addition of a muzzle brake, which increased the submachine gun's overall length by 55 mm. The revised version was designated KP/-31 SJR (suujarru, or "muzzle brake"). Aimo Lahti was displeased with this revision, believing that it decreased muzzle velocity and reduced the weapon's reliability, and even sought in vain to have the muzzle brake's designer court-martialed. Ultimately, roughly half of the KP/-31s in Finnish service were of the SJR version. Initially the KP/-31 was issued as a substitute for a light machine gun, and proved inadequate in this role. Instead, soldiers learned by trial and error how to use submachine guns to the best effect. By the time of the Continuation War, Finnish doctrine had been altered to include both a KP/-31 and a light machine gun (usually a captured Degtyaryov DP) in every infantry squad, and by 1943 this had been expanded to two KP/-31s per squad. KP/-31 production continued with the intention of adding a third submachine gun to each squad, but this plan was shelved in 1944 when the Continuation War ended.

The Suomi KP/-31 also incorporated a few new design features, including an arrangement whereby the spring was mounted inside the bolt in order to make the gun shorter. Its 50-round quad-column "Casket" box magazine was more reliable than the early 40-round "bullets loaded nose down" drum magazine, and similar applications were used on the Argentinian C-4 submachine gun and present-day 60-round 5.45×39mm AK-74 compatible magazines.

A specialized bunker version was also produced in very small numbers (a total of 500 built) in 1941, the barrel shroud end of which was thinner and flattened to allow firing through the narrow ports of defensive bunkers.[5] This version had no shoulder stock and was equipped with a pistol grip. An even rarer version "900 kp 31 psv"[5] was produced for use as a secondary gun in the firing ports of Vickers Alt B Type E 6-Ton tanks, but only a few dozen were built before production was canceled due to the outbreak of the Winter War. Production never resumed, as captured Degtyaryov DP machine guns proved far superior in this role. Like the bunker version, the tank version had a pistol grip and no buttstock, and it could be quickly removed from the tank and fitted with a standard barrel shroud for infantry use if needed. The tank version remained in the Finnish Army's inventory through the 1980s, despite the tank it was designed for being retired in 1959.

In 2009, a semi-automatic version of the Suomi KP was produced for civilian sale in the United States,[6] replacing the receiver and lengthening barrel to meet the standards of the National Firearms Act.

Nazi Germany: 3,042 were ordered by Germany from Finland during The Second World War and likely issued to the Wehrmacht and SS. Also 120 Suomi KP's were presented to the German troops of AOK Norwegen in 1942 for use on the Finnish Front. Most of these weapons left Finland with German troops in 1944 and were subsequently used in other theaters of the war.

Sweden: 35,000 made under license by Husqvarna Vapenfabriks AB just before and during World War II, some were also imported from Finland. Designated kpist m/37 (9mm Browning Long caliber) after the year of adaption. In 1939 Sweden decided to switch to 9 mm Parabellum and the new weapons were designated kpist m/37-39.[7]